Atlfast is the Atlas Fast Simulation package. As shown by the diagram on the right, Atlfast replaces the full detector simulation and reconstruction phases of the Monte Carlo (MC) reconstruction chain. Fast simulation is performed by smearing the MC truth information directly with resolutions measured in full simulation studies.
Atlfast can run using any generator that works in Athena and delivers output in ntuple format. The AtlfastConverters package can also be used to create AODs containing the smeared objects. This is described in the User Guide. While the speed at which Atlfast runs depends on many factors (available CPU, output file format, input file format, complexity of physics channel, etc..), it is in general 4 or 5 orders of magnitude faster than running the full chain.
Click the Status button to see what has changed in recent releases. You can see the Atlfast EDM here.